Photovoltaics (PV) is the only true portable and renewable source of energy available today. Typically, solar cells generate electricity by converting light energy into electricity through excitons. When light is absorbed an electron is promoted from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) forming an exciton. In a PV device, this process must be followed by exciton dissociation to form an electron and a hole. The electron must then reach one electrode while the hole must reach the other electrode in the presence of an electric field in order to achieve charge separation. Generally, the electric field is provided by the asymmetrical ionization energy/workfunctions of the electrodes. The materials and the architecture of solar cell devices should enable and facilitate charge separation and migration of the excitons. However, the lifetime of migrating excitons is extremely short and, as such, an exciton can typically diffuse only a short distance, i.e., about 10 nm to about 100 nm, before the electron recombines with the hole it left behind. To separate the electron away from the hole with which it is bound an electron must reach a junction of an electron accepting material, i.e., a material with higher electron affinity, before the electron recombines with the hole it left behind. Thus, the electron accepting material should be positioned within a migration distance of where the electron originated. Because the primary exciton dissociation site is at the electrode interface, this limits the effective light-harvesting thickness of the device and excitons formed in the middle of the organic layer never reach the electrode interface if the layer is too thick. Rather the electrons recombine as described above and the potential energy is lost.
The efficiency of solar cell devices is generally related to the organization or structure, on a nano-scale, of the materials that make up the solar cell. Inexpensive organic solar cells devices have low efficiency because excitons do not dissociate readily in most organic semiconductors. In order to favor exciton dissociation, the concept of heterojuction was proposed, which uses two materials with different electron affinities and ionization potentials. In order to obtain effective light harvesting and exciton dissociation, bulk heterojunction (BHJ) was employed where the distance an exciton must diffuse from its generation to its dissociation site is reduced in an interpenetrating network of the electron donor and acceptor materials. However, although this conceptual framework has been proposed in the art, the lack of control over nano-scale morphology and structure results in random distribution of the donor and acceptor materials that lead to charge trapping in the conducting pathways.
Several methods have been used to make BHJs, such as: control of blend morphology through processing conditions; synthesis of donor-acceptor copolymers; use of porous organic and inorganic films as templates; self organization; and cosublimation of small molecules to from graded donor-acceptor heterostructures. Such methods are described further in: C. J. Brabec, Solar Energy Materials & Solar Cells 83, 273 (2004); H. Spanggaard, F. C. Krebs, Solar Energy Materials & Solar Cells 83, 125 (2004); F. Yang, M. Shtein, S. R. Forrest, Nature Materials 4, 37 (2005); J. Nelson, Current Opinion in Solid State and Materials Science 6, 87 (2002); and N. Karsi, P. Lang, M. Chehimi, M. Delamar, G. Horowitz, Langmuir, 22, 3118 (2006); each of which is incorporated herein by reference in its entirety. However, due to immiscibility of solid state materials, as well as limited synthesis methods and high cost, these methods result in a lack nano-scale morphology and structural control. Furthermore, current methods of PV fabrication that attempt to control nano-scale morphology fail to produce the desired uniform structures and restrict the overall size or footprint of the photovoltaic cell to roughly one square millimeter and cannot be used for large area device fabrication.
Thus, there is a need for solar cells that have deliberate or predetermined nano-scale morphology, can be fabricated from virtually any material, and that can be fabricated in overall dimensions greater than a few square millimeters.